It's been a while, so my memory is a bit foggy...
Somewhere around the brake master cylinder, under the hood, should be a large wiring harness running through the firewall into the cabin. One option (not the only one) is to sneak a cable in through there. You won't be able to get the wire in with the rest of the harness, but what you can do is to make a new hole between the existing wiring harness and the outer edge of the grommet.
Not an ideal way to do it, but it'll work. You should seal up around the wire when you are done.
Usually, making a small incision with a small knife, like an Exacto hobby knife, works well. Using a long blunt rod (so as not to damage the existing wiring, push through the new hole. It might take some work. Once you get that through, you can use it to pull your new wire in.
Like I said, not the best way to do it, but it'll work.
Other option is to drill a new hole. This can be tricky as I these trucks, I think, had a double walled fire wall. If you do this, it is very important that you install grommets in any holes you drill through sheet metal.
Other places to look on those trucks:
Since they share common parts with the pickups, you may be able to find places where the clutch linkage when through the fire wall.
Other penetrations under the cab, especially around where a manual transfer case shift lever may be.
Best thing you can do is to get under/around/over the vehicle with a bright light. It may take a while, but there are paths.